Abstract

A. R. Wyatt writes: Algeo & Wilkinson (1991) propose 'hypsometric slope' as a measure of the rate of change of elevation with respect to fractional area. They suggest that modern continents show relatively uniform hypsometric slopes over an elevation range of –50 m to +250 m with respect to present-day sea level, and that as these slopes appear be related to continental area, the concept of hypsometric slope can be applied to past continental distributions. I show below that hypsometric slopes can vary considerably over the elevation range –50 m to +250 m. They also state that modern continental hypsometries have been employed without modification in all hypsometric studies of palaeo-eustasy known to them, implying that theirs is the first work to model past hypsometries. I do not believe that this is so, and suggest that use of the normalized hypsographic curve not only allows estimates of areal changes over greater sea-level changes than use of the hypsometric slope, but also constrains predictions rather more precisely.

Hypsometric slope. How is area to be measured for calculations of hypsometric slope? Algeo & Wilkinson take present day sea level as their base, as they are calculating the percentage area flooded for a sea-level change from 0 m to +200 m. But present-day sea level has no particular geological significance, particularly as continents show tectonic as well as eustatic sea-level changes, such as the Tertiary uplift of Africa (Bond 1978). The edge of the continental shelf may be the best marker, but this